Many commercial production operations involve processing steps directed to the removal of byproducts or contaminants from a product mixture. An example of this occurs in the commercial process for producing soybean oil.
Soybean oil is usually produced through either a mechanical pressing process or a solvent extraction process. Various preparative steps can occur in the process including cleaning, drying and dehulling of the soybeans. After the soybeans are prepared, they can be heated in order to coagulate the soy proteins to make the oil extraction process easier. In the solvent extraction process, the soybeans are then cut into flakes which are put in percolation extractors and immersed in a solvent, such as hexane. The soybean oil is solvated by the solvent and then separated from the flakes. The solvent is then separated from the soybean oil, such as with an evaporator. The solvent free crude soybean oil is then further purified (refined). In the mechanical pressing process, the soybeans are typically crushed and pressed to yield a crude soybean oil which is then further purified.
Purification steps in both mechanical pressing processes and solvent extraction processes can include degumming (removing of phosphatides), alkali refining (washing with alkaline solution to remove free fatty acids, colorants, insoluble matter and gums) and bleaching (with activated earth or activated carbon to remove color and other impurities). The alkali refining process (sometimes referred to as caustic refining) is frequently carried out by mixing a strong base, such as sodium hydroxide, into an aqueous solution and then combining the aqueous solution and the crude soybean oil and agitating the mixture. Free fatty acids in the crude soybean oil react (saponification) with the strong base to form a soap composition which is soluble in the aqueous phase and mostly insoluble in the oil phase. The aqueous phase is then removed from the soybean oil carrying away the soap composition.
Alkali refining processes are effective for the removal of free fatty acids, but suffer from various disadvantages. First, alkali refining processes generally require the handling of very caustic strong bases, such as sodium hydroxide, creating safety issues and causing significant wear on equipment. Second, such strong bases can be relatively expensive to use.
It will be appreciated that removal of free fatty acids is a process step that is part of many other industrial production processes. By way of example, biodiesel fuel is produced through a transesterification process that converts triglycerides into fatty acid alkyl esters. This process is frequently carried out with either a strong base or strong acid. In the context of the transesterification process processed with a strong base, the fatty acids react with the strong base to form a soap composition. Unfortunately, significant amounts of fatty acids are frequently present in biological feed stocks used to make biodiesel. As such, even after the transesterification reaction takes place, there can sometimes still be significant amounts of fatty acids present in the product mixture. However, guidelines for biodiesel fuel place strict limits on the concentration of fatty acids that can be present. As such, fatty acids need to be removed from biodiesel fuel made from certain types of feedstocks.